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Air compressor retrofit

08-18-2011, 09:06 PM

One of my clients has a 4.5 Hp, 60 gallon tank air compressor from 1997, one of those oil free types that will drive you nuts in 30 seconds or less. It shattered the connecting rods in the compressor section, probably from over-heating and dirt since the crankcase is open.

Since he isn't flush with cash, he was in a bind. Well, Harbor Freight has component compressors which are near copies of IRs and the reviews are mostly positive. The price is right if the motor can be saved.

Yesterday, I hacked the compressor crankcase with a recip saw, retaining the motor mounts. The bearings are fine so we proceeded.

I built a frame from 1" square tube to hold the new compressor and motor, making a sliding platform from 1-1/2" square tube, flat bar and two 1/2 fine thread bolts for belt tension adjustment. TSC had the necessary pulley (diameter of bore and OD). Sure, it was design as I went but we got it mounted.

Some of the tricks I added were 1/2" steel tube bolt hole liners, autogenously welded to the tubes. Instead of welding to the tank's 12 gauge bracket, I used one existing hole and drilling two more 3/8" holes which are also two of the compressor's mounting bolts.

He has a 14" abrasive chop saw, I managed to make beautiful miter joints, tacking carefully and just needing a bit of 0.023" ER70-S6 filler.

Sure, he could go get a new compressor+tank for $500 but it would still be noisy, this one you can actually carry on a conversation next to it.

Tomorrow, we finish the plumbing. The old one had 3/8" tube for the compressor outlet, the new one uses 1/2". Can you say "choked flow"?

Comment

Pictures are coming...the server is unreachable at the moment...grrr..they have had issues with malware.

Anyhow, this morning we exchanged the 47 inch belt for a 44, perfect fit. Tension adjustment worked very well, didn't need fine thread bolts but it worked.

Plumbing was a bit hairy, the tank check valve has a 3/8" compression, the pump a 1/2" compression. This is a bit of a bottleneck so we used about 3 feet of 1/2" tube in gentle bends for a bit more volume to moderate the compressor impulses. This also makes for better unloading, when the compressor cycles off, an unloader depresses a Schraeder valve to drain the compressor to tank check valve. More volume here means the pump starts unloaded, reducing the load on the start capacitor.

Once the electrical was connected and double checked, it was cautiously plugged in and then the switch turned to on. Runs perfectly, quiet and much faster than the original direct drive. Cooler too as the 12 inch pulley-fan makes a good breeze running at 950 RPM.

You don't know how much you depend on compressed air until it breaks down. And quiet air is always better. Before, hearing protection was required in the shop, now you can talk on the phone right next to the compressor.

We are all very happy with the results for under $200 in used materials. We had to buy 10 feet of copper tube, it doesn't spoil and handy to have.

Here they are:

The first is the frame, about 7" x 16", 1" square tube (old recycled stuff from earlier project). Holes are for the compressor, drilled 1/2" an lined with 1/2" tube (0.065" wall). Bolt holes for the compressor are 8mm, bolts used are 5/16" grade 8. Not shown is the 1/4" thick bar, 2" wide in the middle of the compressor footprint for a 5/8" bolt to the existing compressor mount on the tank. The two bolts in the middle of the frame also secure it to the bracket. It is rock solid mount to the tank. All GTAW welds with ER70-S2 filler for all but outside corners, used S6 for that due to sharp edge and I have thinner S6.

None of us are strong enough to lift both compressor and motor so it was assembled in pieces. That is the second photo. Compressor dry is 52 pounds.

Third photo shows motor and compressor mounted but not plumbed or connected electrically.

Fourth photo shows the compressor sitting on the shop floor. I must have moved that thing fifty times or so my back feels.

Comment

Your tip off the old compressor was going bad was the fact it was so loud ...now you know it was telling you to prepare for a project. Did you check the tank for internal rust while you were at it? The reason I ask is because now that it's fixed you will probably be so happy that you it works so well that you wont look at it for preventative maintenance for a while. Good work and an interesting repair.

Comment

Yes, I looked at the bottom tank head (2:1 elliptical) through the top port. Nothing flaky, just light surface rust. The owner has been draining the water daily.

It is a 1997 ASME Section VIII, Div 1 U stamp so it is a bit more conservative with regards to maximum allowable working pressure. IIRC, it was 1998 when the higher allowable stresses kicked in. One key to this, other than the date, is the 600 F maximum design metal temperature.

Still, this is a thin vessel so your concerns are valid. I know ASME code for pressure vessels, that is why I didn't do any modification or welding to the pressure boundary. In fact, the only modification are two 5/16" holes drilled for mounting. A third 5/8" bolt goes through an existing hole in the bracket.